Selecting write fault thresholds by head and disc location in a disc drive

ABSTRACT

A method is disclosed for optimizing write performance of a disc drive through the selection of optimum write thresholds. The disc drive has a controllably positionable head adjacent tracks defined on a recording surface of a rotatable disc. A narrow head detection test is first applied whereby data are written to a selected track while maintaining the head at a first off-track center distance. The data are read while maintaining the head at a position nominally over the center of the selected track and measuring resulting read error performance. These steps are then repeated using a second off-track center distance. A wide head detection test is next applied, by writing data to the selected track while maintaining the head over the center of the selected track, writing data to an adjacent track abutting the selected track while maintaining the head at a third off-track center distance, and reading the data while maintaining the head over the center of the selected track and measuring resulting read error performance. These steps are then repeated using a fourth off-track center distance. The optimum write thresholds are thereafter selected from the measured read error rates.

RELATED APPLICATIONS

This application claims priority to United States ProvisionalApplications No. 60/088,058 entitled WRITE FAULT THRESHOLDS ADAPTIVE BYRADIUS, and 60/088,051 entitled ADAPTIVE WRITE FAULT THRESHOLDS BY HEAD,both filed Jun. 5, 1998, and is related to U.S. patent application Ser.No. 09/161,105 entitled WRITE FAULT THRESHOLDS ADAPTIVE BY HEAD AND DISCLOCATION IN A DISC DRIVE, filed on even date herewith.

FIELD OF THE INVENTION

This invention relates generally to the field of disc drive storagedevices, and more particularly, but not by way of limitation, toimproving data transfer performance of a disc drive using write faultthresholds that are adapted to each particular head and disc location.

BACKGROUND OF THE INVENTION

Disc drives are digital data storage devices which utilize one or morerotatable, magnetic discs to store and retrieve computer data. Aplurality of controllably positionable read/write heads are used toselectively magnetize tracks on the disc surfaces to store the data, andto transduce the selective magnetization of the tracks to retrieve thedata to a host computer in which the disc drive is mounted.

Typically, each track includes a number of servo fields which areperiodically interspersed with user data fields. The user data fieldsare used to store computer data and the servo fields store prerecordedservo information used by a disc drive servo system to control theposition of the heads.

The servo system operates in two primary modes: seeking and trackfollowing. During a seek, a selected head is moved from an initial trackto a destination track on the corresponding disc surface. Generally, avelocity profile defines a desired velocity trajectory for the head asthe head is first accelerated and then decelerated to move from theinitial track to the destination track. As the head nears thedestination track, a settling mode is entered whereby the servo systemattempts to settle the head onto the destination track in a minimumamount of time. Thereafter, the servo system switches to the trackfollowing mode of operation so that the head is maintained over thedestination track until a subsequent head switch or seek operation isperformed.

Each head includes a write element to write the data to the tracks and aread element which reads the user data and the servo data from thetrack. A typical write element comprises a thin film inductive coilhaving a write gap that, when subjected to a time-varying write currentindicative of the data to be stored, generates a correspondingtime-varying magnetic field across the gap which selectively magnetizesthe tracks. A typical read element construction includes amagneto-resistive (MR) material characterized as having a changedelectrical resistance when subjected to a magnetic field of selectedorientation. Stored data are recovered by passing a read bias currentthrough the read element and detecting changes in voltage thereacross inresponse to the magnetization of the tracks. Although head constructionscan vary, the effective size of the write element is typically larger(with respect to track width) than the size of the read element, and theeffective centers of the read and write elements may be physicallyoffset within the head.

To maintain data integrity and high data transfer rates, it is criticalthat the read and write elements be respectively maintained as close aspracticable over the center of each track during read and writeoperations. For example, even if data are properly written in a centeredrelationship over a selected track, attempting to subsequently read thedata while the head is positioned a sufficient distance away from thecenter of the track may result in an unacceptable number of read errors,due to the inability of the read element to properly detect the writtendata, as well as the potential interference from the selectivemagnetization of an adjacent track. More significantly, writing data toofar away from the track center can prevent subsequent recovery when thehead is centered over the track, and can also corrupt data stored on theadjacent track.

Thus, disc drives typically utilize read fault and write faultthresholds to minimize the occurrence of read errors and dataoverwriting. These thresholds are usually expressed as a percentage oftrack width and define zones about the center of the tracks in whichsafe reading and writing can take place. For example, a typical readfault threshold might be established at ±10% of the track width, so thatread operations are enabled only while the head is less than 10% of thetrack width away from the center of the track. Similarly, a typicalwrite fault threshold might be established at ±17%, so that writeoperations are enabled only while the head is less than 17% away fromthe center of the track. During read and write operations, the servosystem continually monitors the position of the respective elements andcauses the interruption of the respective operation if the threshold isreached or exceeded. The thresholds are determined during disc drivedesign and are intended to balance various factors including trackdensity, acceptable read error rates, expected variations in the sizesof the read and write elements, and acceptable data transfer rates.

It will be recognized that extended read and write operations ofteninvolve the accessing of multiple tracks by a single head, and canfurther involve the accessing of multiple tracks by multiple heads. Inorder to maximize data transfer performance of a disc drive, it isdesirable to begin reading or writing data as soon as the head issufficiently settled onto each accessed track. In practice, disc drivestypically monitor the position of the head as it is settled onto eachtrack and initiate the respective read or write operation as soon as thehead is within the respective fault threshold (and the head is over thedesired user data field).

Hence, while tightening the read and write fault tolerances of a discdrive would likely result in corresponding improvements in error rateperformance by the drive, it would also undesirably degrade the transferrate performance of the drive, as the drive would have to wait longer toensure the head is sufficiently settled onto the destination track (andmaintained in proper relation thereto) before commencing the respectiveread or write operation.

Such tightening of the read and write fault thresholds would also placegreater strains upon the servo system in maintaining the heads withinthe defined acceptable read and write zones, likely resulting in agreater number of interruptions in the data transfer process as read andwrite faults are declared and resolved. Tightening the read and writefault thresholds would also generally result in a reduction of theoperational shock performance characteristics of the drive, as the drivewould be less tolerant to the application of external shocks andvibrations that tend to move the heads away from the centers of thefollowed tracks.

Consumer demand for disc drives with ever increasing data storagecapacities and transfer rate performance levels has led disc drivemanufacturers to attempt to achieve greater data storage densities andread/write channel capabilities in successive generations of drives,including balancing the sometimes conflicting requirements of enhancederror rate and transfer rate performance. Accordingly, it is to thefurtherance of these efforts to improve disc drive performance that thepresent invention is directed.

SUMMARY OF THE INVENTION

The present invention provides an apparatus and method for optimizingdisc drive data transfer rate performance through the use of writethresholds that are adaptive by head and by disc location.

As exemplified by a preferred embodiment, a disc drive comprises a headwhich is controllably positionable adjacent each of a plurality ofnominally concentric tracks defined on a rotatable disc, whichselectively magnetizes the tracks to write data to the tracks.

A narrow head detection test is first applied whereby data are writtento a selected track while maintaining the head at a first off-trackcenter distance. The data are read while maintaining the head at aposition nominally over the center of the selected track and measuringresulting read error performance. These steps are then repeated using asecond off-track center distance. A wide head detection test is nextapplied, by writing data to the selected track while maintaining thehead over the center of the selected track, writing data to an adjacenttrack abutting the selected track while maintaining the head at a thirdoff-track center distance, and reading the data while maintaining thehead over the center of the selected track and measuring resulting readerror performance. These steps are then repeated using a fourthoff-track center distance. The optimum write thresholds are thereafterselected from the measured read error rates.

These and various other features as well as advantages whichcharacterize the present invention will be apparent from a reading ofthe following detailed description and a review of the associateddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a top plan view of a disc drive constructed inaccordance with a preferred embodiment of the present invention.

FIG. 2 provides a functional block diagram of relevant portions of thedisc drive of FIG. 1.

FIG. 3 shows a functional block diagram for the communication circuitand servo circuit of FIG. 2.

FIG. 4 shows a representation of a portion of a selected track on thedisc of FIG. 1.

FIG. 5 shows a representation of a selected servo field from FIG. 4.

FIG. 6 shows the general configuration of the position fields of FIG. 5for a number of adjacent tracks on the disc.

FIG. 7 shows write thresholds at selected radial distances from thecenter of a selected track from FIG. 6, the write thresholds selected inaccordance with preferred embodiments of the present invention.

FIG. 8 provides a graphical representation of a velocity trajectorytaken by the head at the conclusion of a seek operation to a destinationtrack.

FIG. 9 provides a flow chart for a WRITE THRESHOLD SELECTION routine,representative of programming stored in memory and used by the systemprocessor of FIG. 2 to set the write thresholds for the tracks inaccordance with preferred embodiments of the present invention.

FIG. 10 provides a graphical representation of error rate data obtainedfrom the routine of FIG. 9.

FIG. 11 is a graphical representation of write thresholds for threedifferent heads of the disc drive.

FIG. 12 provides a flow chart for a WRITE OPERATION routine, indicativeof the steps performed by the disc drive during a write operation usingthe adaptive write thresholds determined by the routine of FIG. 9.

FIG. 13 shows a portion of a read channel similar to that shown in FIG.3, illustrating a quality monitor which can be used to obtain datauseful in selecting optimum write thresholds for the tracks of the discdrive.

DETAILED DESCRIPTION

In order to set forth a detailed description of a preferred embodimentof the present invention, reference is first made to FIG. 1 which showsa top plan view of a disc drive 100 used to store computer data. Thedisc drive 100 includes a head-disc assembly (HDA) 101 and a printedwiring assembly (PWA) which is mounted to the underside of the HDA (andthus not visible in FIG. 1). The PWA includes electronics used tocontrol the operation of the HDA 101, as discussed below.

A top cover, omitted from FIG. 1 to reveal interior portions of the HDA101, mates with a base deck 102 of the HDA 101 to provide anenvironmentally controlled housing. A spindle motor (generallydesignated at 104) is supported by the base deck 102 and rotates aplurality of discs 106 at a constant high speed. A disc clamp 108secures the discs 106 to the spindle motor 104.

The discs 106 include recording surfaces (not separately identified) towhich user data are written by way of a rotary actuator assembly 110,which rotates about a cartridge bearing assembly 112 in response to theapplication of current to a coil (113, a portion of which is visible inFIG. 1) of a voice coil motor (VCM) 114. A plurality of rigid arms 116extend from the actuator assembly 110, each of which supports acorresponding flexible suspension assembly 118. A plurality of heads 120are supported by the suspension assemblies 118 over the recordingsurfaces of the discs 106 by air bearings established by air currentsset up by the high speed rotation of the discs 106. The heads 120 arepreferably characterized as magneto-resistive (MR) heads, each having athin film inductive write element and an MR read element.

A latch assembly 121 secures the actuator assembly 110 when the discdrive 100 is deactivated, and a flex circuit assembly 124 facilitateselectrical interconnection between the actuator assembly 110 and thedisc drive PWA.

Referring to FIG. 2, shown therein is a generalized functional blockdiagram of relevant portions of the disc drive 100 of FIG. 1, includingcircuitry disposed on the aforementioned disc drive PWA. The disc drive100 is shown to be operably coupled to a host computer 126 in which thedisc drive 100 is mounted.

A system processor 130 provides top level control of the operation ofthe disc drive 100. Programming and parameter values utilized by thesystem processor 130 are stored in drive processor memory 132 (MEM),which preferably comprises both volatile and non-volatile memory devicessuch as dynamic random access memory (DRAM) and flash memory. Aninterface circuit 134 includes a data buffer (not shown) for thetemporary buffering of transferred data, and a sequence controller(“sequencer”, also not shown) which directs the operation of the discdrive 100 during data transfer operations.

FIG. 2 further shows a communication circuit 136 operably coupled to theinterface circuit 134 and to the head 120, with the communicationcircuit 136 controlling the transfer of data between the discs 106 andthe host computer 126. A servo circuit controls the radial position ofthe head 120 through the controlled application of current to the coil113.

FIG. 3 provides a functional block diagram of the communication circuit136 and the servo circuit 138 of FIG. 2. The communication circuit 136includes both a write channel (generally designated at 140) to controlthe storage of data to the discs 106 and a read channel (generallydesignated at 142) to control the retrieval of the data from the discs106 back to the host computer 126.

The write channel 140 comprises an encoder 144 which, upon receipt ofdata from the interface circuit 134, encodes the data with runlength-limited and error correction code (ECC) encoding to facilitateclock recovery and recovered data integrity. The encoded data areprovided to a write controller 146, which serializes the encoded data togenerate control signals used by a preamp/driver circuit 148 to apply atime-varying write current to the head 120 to write the encoded data tothe disc 106. The preamp/driver circuit 148 is located within the HDA101, mounted to the actuator 110 as shown in FIG. 1.

The read channel 142 receives readback signals from the head 120 which,after preamplification by the preamp/driver circuit 148, are provided bya multiplexor 150 to an automatic gain control (AGC) circuit 152, whichcontrollably adjusts the amplitudes of the signals to a levelappropriate for remaining portions of the read channel 142.

The signals output by the AGC circuit 152 are converted to a sequence ofdigital samples using an analog-to-digital (A/D) converter 154. A finiteresponse filter (FIR) 156 filters the digital samples to a selectedclass of partial-response, maximum likelihood (PRML) filtering, such asEPR-4. A Viterbi detector 158 decodes the original encoded sequence fromthe FIR 154 and a postcoder 160 removes the RLL encoding. Finally, anerror correction code (ECC) circuit 162 applies on-the-fly errordetection and correction to output the originally stored data to theinterface circuit 134 (FIG. 2), for subsequent transfer to the hostcomputer 126. For reference, the operation of the ECC circuit 162 cantake place in the interface circuit 134 (FIG. 2).

FIG. 3 further shows the servo circuit 138 to include an AGC 164 which,like the AGC 152, controllably adjusts the amplitudes of readback servosignals read by the head 120 to a level appropriate for remainingportions of the servo circuit 138. A demodulator circuit 166 conditionsthe readback servo signals, including conversion to digital form, forprocessing by a digital signal processor (DSP) 168.

The DSP 168 controls the operation of the servo circuit 138 in responseto commands issued by the system processor 130 (FIG. 2). During a trackfollowing mode of operation, the DSP 168 generates a position errorsignal (PES) indicative of the position of the head 120 relative to thefollowed track and, in response to a desired position for the head 120,outputs a current command signal to a coil driver 170 which adjusts theamount of current applied to the coil 113 to maintain the head in adesired relation with the track. During a seek, the DSP 168 appliescurrent to the coil to first accelerate and then decelerate the head 120from an initial track to a destination track in accordance with avelocity profile, indicative of the desired velocity trajectory for thehead during the seek. Programming for the DSP 168 is provided in DSPmemory (MEM) 172.

FIG. 4 provides a schematic representation of a portion of a selectedtrack 174 from the disc 106 (FIG. 1), to illustrate the general mannerin which information is stored on the discs. As shown in FIG. 4, thetrack 174 includes periodically disposed servo fields 176 containingservo information utilized by the servo circuit 138. The servoinformation is written during disc drive manufacturing using a servotrack writer and is generally arranged on each of the discs 106 asradially extending wedges, like spokes on a wheel.

Between each pair of adjacent servo fields 176 is a user data field 178,wherein one or more data sectors are defined during a disc driveformatting operation. User data are thereafter stored to these sectorsby the disc drive 100.

FIG. 5 provides a view of one of the servo fields 176 in greater detail.The servo field 176 includes an AGC field 180 which includes a 2Trepeating timing pattern to enable the AGC 164 of the servo circuit 138(FIG. 3) to adjust its amplitude to an appropriate level. Asynchronization field (sync) 182 includes a unique pattern which enablesthe servo circuit 138 to identify the servo information. An index field184 identifies the angular position for the servo field 176 and the Graycode (GC) field 186 identifies the radial position for the servo field176 by indicating a track address unique to each track. A position field188 enables the DSP 168 (FIG. 2) to determine the radial position of thehead 120 relative to the followed track, as set forth more clearly inFIG. 6.

FIG. 6 shows the position fields 188 of the servo fields 176 to includea number of position fields, or dibits, preferably arranged in aquadrature (off-set checkerboard) pattern and comprising a repeatingseries of A, B, C and D fields 190, 192, 194 and 196, respectively. Asset forth by FIG. 6, the A and B fields 190, 192 extend from adjacenttrack centers and the C and D fields 194, 196 extend from adjacent trackboundaries. Hence, from the relative magnitudes of the readback signalsfrom the A, B, C and D signals, the DSP 168 can determine the relativeposition of the head 120 with respect to a particular track beingfollowed. For reference, tracks containing a C field 194 are referred toas “even” tracks and tracks containing a D field 196 are referred to as“odd” tracks. The direction of disc rotation with respect to the head120 is indicated by arrow 200, and track boundaries demarcated byadjacent C and D fields 194, 196 are numerically identified as 1-4.

To minimize the interference of data written to adjacent tracks, writefault thresholds T₁, T₂ (also referred to as “write thresholds”) areidentified for each fault, as shown in FIG. 7 and identified at 202,204, respectively. The write fault thresholds T₁, T₂ are determined foreach track in a manner discussed below to indicate the maximum radialdistance (in terms of percentage off-track from track center 206) thehead 120 can depart from track center and still safely write data to theparticular track.

During a track following mode of operation wherein the head 120 iscaused to follow the center of a selected track, the servo circuit 138continually monitors the relative position of the head and declares awrite fault at such time that the head moves to a position beyond thewrite thresholds T₁, T₂, thereby preventing the sequencer from enablingthe communication circuit 136 to initiate a write operation to the discs106 (or causing the interruption of an on-going write operation).Likewise, as shown by FIG. 8, when a write operation is to occur upon adestination track accessed at the conclusion of a seek to thedestination track, the head generally follows a settle profile as setforth by FIG. 8 by head velocity trajectory 208, plotted against a timex-axis 207 and a position y-axis 209. Thus, the servo circuit 138prevents writing to the destination track until such time that the headhas been settled upon the destination track within the associated writefault thresholds shown in FIG. 7.

The manner in which the write thresholds are selected for each of thetracks on the discs 106 will now be set forth by FIG. 9, whichillustrates a WRITE THRESHOLD SELECTION routine 210 indicative ofprogramming stored in MEM 132 and utilized by the system processor 130in controlling the operation of the servo circuit 138. It will berecognized that the routine of FIG. 9 is desirably performed during discdrive manufacturing, but can also be performed from time to time duringfield operation of the disc drive 100 to maintain optimal operation ofthe drive.

As shown in FIG. 9, the first step involves the identification of aselected track for which the write thresholds T₁, T₂ are to bedetermined, as indicated by step 212. At this point it will beunderstood that the routine 210 is preferably performed for a selectedtrack from each of a plurality of constant bit-density zones definedupon the discs using conventional zone based recording (ZBR) techniquessuch as discussed in U.S. Pat. No. 4,799,112 issued Jan. 17, 1989 toBremmer et al., assigned to the assignee of the present invention. WhenZBR is employed, the tracks are grouped into a plurality of zones witheach of the tracks in each zone having a common data storage capacity(i.e., the same number of data sectors). Thus, in a preferredembodiment, each of the tracks in each zone will be assigned the samewrite thresholds. Of course, different zones will have different writethresholds.

Alternatively, as desired the routine of FIG. 9 can be individuallyperformed for each track on each of the recording surfaces of the discs106, or can be performed on a track at the inner and outer diameters ofthe discs 106 to allow the use of subsequent interpolation techniques todetermine appropriate write thresholds for each of the intermediarytracks. In this case each track will have its own set of writethresholds.

Regardless, once the selected track is identified by step 212, theroutine of FIG. 9 initiates what is referred to as a “narrow head” test,to evaluate whether the head 120 is a relatively narrow head (that is,whether the effective width of the head can be considered to berelatively small as compared to the width of the tracks). At step 214,an initial off-track center (OTC) level is selected, such as 10%. Theroutine continues at step 216 wherein the head is positioned at theselected OTC level and test data are written to selected user datafields (178, FIG. 4) of the selected track by the head 120. Once thistest data are written, the routine next positions the head 120 back overthe track center (TC, such as 206, FIG. 7) of the selected track andattempts to read the test data, as indicated by step 218. During thereading of this test data (which desirably occurs over a number ofrevolutions of the disc 106), an error rate is measured, step 220. Moreparticularly, the read channel 142 (FIG. 3) recovers the test data anddetermines the number of errors detected by the ECC circuit 162.

The routine of FIG. 9 next determines whether additional OTC levelsshould be evaluated at decision step 222. In one preferred embodiment, apredetermined set of OTC levels is identified (such as 10, 12, 14, 16,18 and 20 percent), so that the routine sequentially cycles throughthese increments in OTC level; alternatively, the routine continuesuntil a selected level of error rate is obtained. At such time thatadditional OTC levels are to be evaluated, the routine passes fromdecision step 222 to block 224 wherein the OTC level is incremented andthe routine repeats steps 216 through 220. The resulting error rates aretemporarily stored in MEM 132 (FIG. 2).

Once the narrow head test is completed, the routine of FIG. 9 nextperforms a wide head test, to determine whether the effective width ofthe head is relatively “wide” with respect to the width of the tracks.To do so, the routine continues at step 226 wherein the head 120 iscaused to write test data to the selected track with the head positionedat track center (TC). Next, an initial off -track center (OTC) level isselected, step 228, and the head writes data to adjacent tracks at theselected OTC level in a direction toward the selected track, as setforth by step 230. By way of example, with reference back to FIG. 6 andconsidering the selected track to be the track disposed between trackboundaries 2 and 3 (identified at 232), the adjacent tracks to which thedata are written are the adjacent tracks identified as the tracksdisposed between track boundaries 1 and 2 (identified at 234) andbetween track boundaries 3 and 4 (identified at 236), respectively.

Once the test data have been written to the adjacent tracks, the routineof FIG. 9 continues to step 238 wherein the head 120 reads back the datafrom the selected track that was previously written at step 226. Theresulting error rate is evaluated at step 240 (and the results arestored in MEM 132, FIG. 2).

Decision step 242 determines whether additional OTC levels should beevaluated (in a manner similar to discussed above for decision step222); if so, the routine passes to step 244 wherein the next OTC levelis selected and the steps 230 through 240 are performed again for thenext OTC level. In this way, the effects of data written to adjacenttracks can be evaluated.

Finally, when all of the OTC levels have been selected, the routinepasses from the decision step 242 to step 246 wherein the particularwrite thresholds T₁, T₂ for the selected track are selected based uponthe error rates determined at steps 220, 240, after which the routineends at step 248. The write thresholds are preferably stored in a tablein MEM 172 for subsequent access by the DSP 168 during servo operation,as explained below.

The manner in which step 246 operates to select the final writethresholds T₁, T₂ for the selected track can be better understood from areview of FIG. 10, which shows a graphical representation of the errorrate data obtained from steps 220, 240 of FIG. 9. More particularly,FIG. 10 plots a first error rate curve 250 and a second error rate curve252 against an x-axis 254 indicative of OTC level and a y-axis 256indicative of error rate. In the spirit of presenting a preferredembodiment, dotted line 258 represents an acceptance threshold at 10⁻⁷,which corresponds to a measured error rate of 1×10⁻⁷ errors per bit read(errors/bit). Of course, the particular error rate threshold will beselected from the constraints of a given application and may notnecessarily be the threshold indicated by the line 258.

As shown in FIG. 10, the first error rate curve 250 (which for purposesof illustration is contemplated as arising from the error rate dataobtained by the operation of step 220 of FIG. 9) shows a relatively flatresponse through OTC levels up to 14%, after which a rapid deteriorationin error rate performance occurs. Correspondingly, the second error ratecurve 252 (which is contemplated as arising from the error rate dataobtained by the operation of step 240 of FIG. 9) shows a relativelylinear response for OTC levels from 10% to 22%.

From the curves 250, 252, it will be recognized that the head 120 can becharacterized as a relatively narrow head, in that the head providedrelatively linear, controlled error rate performance for the wide headtest (as indicated by curve 252) but quickly deteriorated during thenarrow head test (as indicated by the upward trend in the curve 250 forOTC levels greater than 14%). In other words, the head is notparticularly affected by data written to adjacent tracks, but begins todramatically lose its ability to reliably recover data from the selectedtrack when the head is moved off-track too far beyond a level of around14%.

From the data obtained from the operation of steps 220, 240 of theroutine of FIG. 9, the step 246 will select write thresholds T₁, T₂ thatensure the head 120 can reliably recover data from the selected track.In a preferred embodiment, the step 246 operates to move in a directionto the right from the y-axis 256 along the predetermined acceptancethreshold 258 until the first curve (in the present example, curve 250)is reached, and the corresponding point on the first curve isidentified; thus, in this case, a value of about 15.6%, as indicated bydotted line 260. Of course, other criteria might be used, such aschanges in slope of the curves 250, 252, to determine the appropriatethresholds T₁, T₂. In this case, the thresholds might be moreconservatively selected at around 14%, the point at which the slope ofcurve 250 drastically increases.

FIG. 11 sets forth the results of write thresholds for three differentheads (substantially similar to the head 120 discussed above) in thedisc drive 100, to illustrate a range of different write thresholds thatmight be obtained for a particular drive. More particularly, FIG. 11shows three different write threshold curves 262, 264 and 266, which areplotted against an x-axis 268 indicative of track position from outerdiameter (OD) to inner diameter (ID) on a selected disc 106 and a y-axis270 indicative of write threshold level.

The first write threshold curve 262 shows variation in write thresholdsfrom about 2% at the OD to about 8% at the ID. As will be recognized, itis questionable whether this particular head possesses sufficient writecharacteristics to warrant use in the drive, and would likely bereplaced during manufacturing of the drive. The second write thresholdcurve 264, intended to represent a moderate response, has minimum writethresholds for tracks at the OD of around 9%, maximum write thresholdsat around 15% for intermediary tracks, and write thresholds of around12% at the ID. This head would likely be used in the disc drive 100 andwould provide acceptable error rate performance, but would also likelyresult in a higher number of declared write faults and slower accesstimes, due to the relatively tight write tolerances required for thehead. Finally, the third write threshold curve 266 exhibits relativelygood performance, allowing use of relaxed write thresholds of around 23%at the OD and about 20% at the ID.

Thus, by individually setting the write thresholds for each head on atrack by track basis, overall data transfer performance of the drive canbe enhanced, by allowing the use of marginal heads that might otherwisefail to meet a standard write threshold and capitalizing on the abilityof certain, good heads to reliably write data at greater writethresholds. The routine of FIG. 9 can also be utilized to identify andreplace marginal heads (such as represented by the curve 262) whichmight not be otherwise detected during disc drive manufacturingscreening processes.

To review the manner in which the individually selected write thresholdsT₁, T₂ are utilized during normal operation of the drive, reference ismade to FIG. 12, which sets forth a flow chart for a WRITE OPERATIONroutine 280, representative of programming stored in MEM 132 andutilized by the system processor 130 (FIG. 2).

At such time that a write operation is desired upon a selected user datafield (178, FIG. 4), the servo circuit 138 operates to position the head120 over the selected track containing the selected user data field,step 282, after which the sequencer (of the interface circuit 134, FIG.2) waits for verification from the servo circuit 138 that the head 120is within the particular write thresholds T₁, T₂ selected for the track.As shown by step 284, the DSP 168 recalls the associated writethresholds T₁, T₂ from MEM 172, determines whether the position of thehead 120 falls within said thresholds, and provides an affirmativeindication to the sequencer when such is the case. The sequencer thencommences the assertion of a write gate signal, enabling the writechannel 140 (FIG. 3) to write the data to the user data field 178, step286. The routine then ends at step 288. Although not explicitly shown inFIG. 12, the sequence correspondingly disables the write operation ifthe write thresholds are exceeded.

Having concluded a review of a preferred embodiment of the presentinvention, additional considerations will now be addressed. First, forread channels employing the use of a quality monitor (such as set forthat 302 in FIG. 13), the output of the quality monitor can be used inlieu of a calculation of error rate as discussed above with theoperation of steps 220, 240 of FIG. 9.

More particularly, FIG. 13 sets forth a portion of a read channel 300generally similar to the read channel 142 of FIG. 3, including a Viterbidetector 158 and a postcoder 160, with the quality monitor disposedtherebetween. As will be recognized by those skilled in the art, theoutput from the Viterbi decoder 158 will optimally comprise a datasequence corresponding to the encoded data originally written to theselected track. This sequence is provided to the channel quality monitor302 which performs a running assessment of the channel quality usingerror values provided by the Viterbi decoder 158. The channel qualitymonitor 302 generates a status byte which represents the integral, orsum, of the square of the sampled data bit error values recovered duringthe read operation. The magnitude of this status byte (“channel qualitymeasurement”) is representative of the overall quality of the datasignal during the read event; the lower the magnitude, the higher thequality of the signal. Because the individual sample errors are squared,larger errors generally carry much more significance than smaller errorsin the overall count.

In a preferred embodiment, the A/D converter 154 (FIG. 3) producessample values over a symmetrical integer range (−18 to +18), and eachsample received by the Viterbi decoder 158 will have one of three valuescorresponding to symbol values of −1, 0 and +1(−14, 0, or +14). Thus,samples other than these will have non-zero error values. Accordingly,the Viterbi decoder 158 provides these error values to the channelquality monitor 302, which accumulates the same in a summing register(identified at Q_(m) in FIG. 13) to generate the channel qualitymeasurements. The summing register Q_(m) can thereafter be periodicallypolled by the system processor 130.

It is advantageous to linearly scale the squared error values beforebeing summed and then to linearly scale the resulting sum, so that themeasurement fits in a single-byte register. When a new sector of data isread, the channel quality monitor 302 typically clears the summingregister during receipt of the associated phase-locked loop (PLL)recovery field and then begins adding the calculated squares of theerror values when data recovery begins. As desired, a root mean squared(RMS) integral of error can readily be determined for the qualitymeasurement by further dividing the sum by the total number of bitsreceived and then taking the square root of this value. It will berecognized that various methodologies for calculating channel qualitymeasurements are well known in the art.

Through experimentation, the relationship between the channel qualitymeasurement and the overall channel read error rate can be readilyidentified.

An advantage of use of the quality monitor 302 is that substantiallyless data need be read to obtain a corresponding assessment of overallerror rate of the channel 300.

Another consideration is the use of non-uniform write thresholds T₁, T₂.

While the foregoing discussion has contemplated each set of writethresholds T₁ and T₂ being disposed at equal radial distances from thecenter of the associated track, it will be readily apparent that theroutine of FIG. 9 can be performed to separately determine writethresholds that are at different radial distances from track center, asdesired. An advantage of using such non-uniform write thresholds T₁, T₂is the potential for further improvements in disc drive operation, butat the expense of requiring additional storage capacity to handle acorresponding larger number of threshold values for the tracks.

In summary, the present invention is directed to a method for optimizingdisc drive write performance through the selection of optimum writethresholds.

As exemplified by a preferred embodiment, a disc drive 100 has a head120 which is controllably positionable adjacent each of a plurality ofnominally concentric tracks 174, 232, 234, 236 defined on a rotatabledisc 106, which selectively magnetizes the tracks to write data to thetracks.

A narrow head detection test is first applied whereby data are writtento a selected track while maintaining the head at a first off-trackcenter distance (step 216, FIG. 9). The data are read while maintainingthe head at a position nominally over the center of the selected trackand measuring resulting read error performance (steps 218, 220). Thesesteps are then repeated using a second off-track center distance (steps224, 216, 218, 220). A wide head detection test is next applied, bywriting data to the selected track while maintaining the head over thecenter of the selected track (step 226), writing data to an adjacenttrack abutting the selected track while maintaining the head at a thirdoff-track center distance (step 230), and reading the data whilemaintaining the head over the center of the selected track and measuringresulting read error performance (steps 238, 240). These steps are thenrepeated using a fourth off-track center distance. The optimum writethresholds are thereafter selected from the measured read error rates(step 248).

It will be clear that the present invention is well adapted to attainthe ends and advantages mentioned as well as those inherent therein.While presently preferred embodiments have been described for purposesof this disclosure, numerous changes may be made which will readilysuggest themselves to those skilled in the art and which are encompassedin the spirit of the invention disclosed and as defined in the appendedclaims.

What is claimed is:
 1. A method for selecting optimum write thresholdsfor a disc drive of the type having a controllably positionable headadjacent tracks defined on a recording surface of a rotatable disc,wherein the disc drive subsequently inhibits writing of data when thehead is disposed at positions beyond the write thresholds, the methodcomprising steps of: (a) writing data to a selected track whilemaintaining the head at a position away from a center of the selectedtrack equal to a first off-track center distance; (b) reading the datawritten in step (a) while maintaining the head at a position nominallyover the center of the selected track and measuring resulting read errorperformance; (c) repeating steps (a) and (b) using a second off-trackcenter distance; and (d) selecting the optimum write thresholds for theselected track from the measured read error performance for the firstand second off-track center distances.
 2. The method of claim 1, furthercomprising steps of: (e) writing data to the selected track whilemaintaining the head at a position over the center of the selectedtrack; (f) writing data to an adjacent track abutting the selected trackwhile maintaining the head at a position away from a center of theadjacent track equal to a third off-track center distance; and (g)reading the data written in step (e) while maintaining the head at aposition over the center of the selected track and measuring resultingread error performance, (h) repeating steps (e) through (g) using afourth off-track center distance; and (i) further selecting the optimumwrite thresholds using the third and fourth off-track center distances.3. The method of claim 2, further comprising: (j) repeating steps (a)through (i) for a second selected track on the disc to obtain optimumwrite thresholds for the second selected track.
 4. The method of claim2, wherein the disc drive is characterized as utilizing zone basedrecording so that the tracks are grouped into a plurality of zones witheach of the tracks in each zone having a common data storage capacity,and wherein steps (a) through (i) are repeated for a selected track fromeach zone to obtain optimum write thresholds for each of the tracks ineach zone.
 5. The method of claim 1, wherein the measuring of theresulting read error performance comprises determining a read error ratefrom read errors detected by an error correction code circuit of a readchannel of the disc drive.
 6. The method of claim 1, wherein themeasuring of the resulting read error performance comprises obtainingquality monitor measurements from a quality monitor of a read channel ofthe disc drive.
 7. The method of claim 1, wherein the disc drive ischaracterized as utilizing zone based recording so that the tracks aregrouped into a plurality of zones with each of the tracks in each zonehaving a common data storage capacity, and wherein steps (a) through (d)are repeated for a selected track from each zone to obtain optimum writethresholds for each of the tracks in each zone.